Vehicles may include a fluid coupling, e.g., a torque converter, which interconnects an output from an engine with an input into a transmission. The vehicle may further include a torque converter clutch that mechanically connects a turbine of the torque converter to a pump of the torque converter. When the rotational speed of the engine is relatively constant and approximately equal to the rotational speed of the turbine, the torque converter clutch may be positioned in a locked operating state to mechanically connect the output of the engine and the input of the transmission to reduce energy losses through the torque converter. At other times, such as during acceleration or braking, the torque converter clutch may be positioned in an unlocked operating state to disconnect the mechanical connection between the output of the engine and the input of the transmission and allow relative slippage between the turbine and the pump of the torque converter.
When the torque converter clutch is in the unlocked operating state, the transmission is not actively controlling the rotational speed of the engine. It is desirable to have the rotational speed of the engine less than the rotational speed of the turbine to ensure that there is enough negative acceleration when the vehicle is coasting to reduce a sail-on effect, and to prevent acceleration during fast braking, which may occur if the rotational speed of the turbine falls below the rotational speed of the engine. During coastdown, the torque output of the engine is controlled to regulate the rotational speed of the engine so that the rotational speed of the engine does not greatly differ from a rotational speed of the turbine of the torque converter. Typically, a control module will adjust certain operating parameters, such as the timing and/or throttle position, to regulate the torque output of the engine to control the rotational speed of the engine. The control module references a table that defines desired values for the certain operating parameters given the specific operating conditions of the vehicle. The control module then adjusts the various operating parameters to achieve the desired engine speed. In so doing, the rotational engine speed may be as much as 200 to 400 rpm below the rotational speed of the turbine to ensure that the rotational speed of the engine is not greater than the rotational speed of the turbine. However, this high lash, i.e., the 200 to 400 rpm difference between the rotational speed of the engine and the rotational speed of the turbine, may cause a clunk or other undesirable noise.